Cathode structures for thermionic valves



May 3, 1960 F. w. R. FARROW 2,935,638

CATHODE STRUCTURES FOR THERMIONIC VALVES Filed June 2. 1959 INVENTOR FREDERICK WILLIAM RODBARD FARROW BY cy w 1d ATTORNEYS 2,935,638 Patented May 3, 1960 CATHODE STRUCTURES FOR THERMIONIC VALVES Frederic William Rodbard Farrow, Enfield, England, asslgnor to Siemens Edison Swan Limited, London, England, a British company Application June 2, 1959, Serial No. 817,575

Claims priority, application Great Birtain June 10, 1958 9 Claims. (Cl. 313--340) This invention relates to indirectly heated cathode structures and is especially applicable to such structures of the kind in which an electrically insulated heating element is accommodated within a metal sleeve coated with electron-emissive material.

\ In one known construction of indirectly heated cathode structure of the kind specified the heating element which takes the form of a length of electrically insulated refractory wire of spiralised hairpin or M-shaped configuration is supported within the sleeve by means of a metal-cored helical insulator which fits into the metal sleeve and embraces the heating element.

The present invention has in view a form of cathode heater which enables the separate helical insulator, just above referred to, to be dispensed with.

According to the present invention a cathode heater such as for an indirectly heated cathode structure of the kind set forth comprises a helical heating element of small pitch coated externally with a layer of refractory electrically insulating material defining an insulating sleeve with an external helical protuberance of relatively large pitch which determines the minimum spacing of said heating element from the cathode sleeve of said structure when the heater is inserted therein.

In order to ensure good heat transference from the heating element to the cathode sleeve so that the time required to raise the latter to its electron-emitting temperature is not unduly long it is desirable to keep the mass of insulating material to a minimum, and with this in view, together with the need to keep the heater to cathode capacity at a low level, the helical protuberance is preferably narrow and of hollow form.

The invention will now be described by way of example wtih reference to the accompanying drawing, in which:

Fig. 1 shows a cathode heater according to the invention; and,

Fig. 2 shows a fragmentary longitudinal cross-sectional view of the heater according to Fig. 1.

In manufacturing the cathode heater illustrated a helical heating element 1 of small pitch (e.g. having 100-500 turns per inch) is formed by winding tungsten wire for example on to a suitable mandrel (not shown) such as of molybdenum, or other material capable of being dissolved in acid without the latter attacking the wire of the heating element. Molybdenum or other wire likewise capable of being dissolved in acid is wound over the element 1 so as to form thereon an outer helix having a relatviely large pitch (e.g. having turns per inch). The mandrel and the two helices carried thereby are then passed through a furnace to set the helices, after which they are cut into the desired lengths. Each length is then bent into the form of a hairpin as shown in Fig. l or otherwise bent according to the desired shape of the heater and thereafter thinly coated with a refractory electrically insulating material defining sleeve 2 such as by spraying, dipping or electrophoresis.

For example where electrophoresis is used the heating element and the outer helix may be coated with refractory insulating material by suspending them in a bath containing a suspension having the following compositions Alumina gms 1980 Silica g 20 Aluminium nitrate gms 48 Magnesium nitrate "gals-.. 45

Water mls- 1300 Methylated spirit mls 1800 with the heating element being connected to the negative terminal of a source of direct current voltage and the bath, if of metal, being connected to the positive terminal of the voltage source. Otherwise a metal anode will be immersed in the bath. The coated helices are then sintered and finally immersed in an acid bath so as to dissolve the mandrel and the outer wire helix. The outer wire helix in dissolving leaves a hollow protrusion 3 of helical configuration on the outer surface of the coated heater and this protrusion will serve to ensure a minimum spacing and insulation between the heating element and the inner cylindrical surface of a cathode sleeve of an indirectly heated cathode structure of. the kind hereinbefore specified when the heater is inserted into the sleeve.

If desired the insulating coating of the heater may be strengthened to render the heater more robust by coating the heater with a bonding agent such as a solution of polymethacrylic ester or nitro-cellulose.

In the manufacture of the embodiment described the mandrel and the wire forming the outer helix may have a diameter of .008 while the diameter of the heating wire and the thickness of the insulating coating may be .003".

From the foregoing it will be apparent that the invention provides a cathode heater which is formed with integral helical spacing means, thereby obviating the need for a separate helical insulator between the cathode sleeve and the heating element.

As well as providing good insulation between the heating element and the cathode sleeve the helical protuberance will, in cases where the heater is a close fit in the cathode sleeve prevent movement of the heater with respect to the sleeve and thereby prevent hum due to variations in the heater to cathode capacitance in valves subject to vibration.

What we claim is:

1. A cathode heater comprising a helical heating element of small pitch coated externally with a layer of refractory electrically insulating material defining an insulating sleeve with an external helical protuberance of relatively large pitch.

2. A cathode heater as claimed in claim 1, in which the helical protuberance is narrow and of hollow form.

3. A method of manufacturing a cathode heater including the steps of winding a helical heating element of small pitch on a mandrel, winding a helix of relatively large pitch on said element, coating the assembly with a layer of refractory insulating material and dissolving the mandrel and the outer helix with acid incapable of attacking the heating element and insulating material.

4. The method as claimed in claim 3, in which the heater is bent into its final shape immediately prior to coating it with the refractory insulating material.

5. The method as claimed in claim 3, in which the refractory insulating material is applied by electrophoresis.

6. The method as claimed in claim 3, in which the refractory material is applied by spraying.

7. The method. as claimed in claim 3, in which the refractory material is applied by dipping.

' messes 8. The method as claimed in claim 3, in which a bonda cathode heater of hairpin configuration as claimed in ing agent such as a solution of polymethacrylic ester or claim I, mounted within a tubular cathode coated with Intro-cellulose is finally applied to the insulating layer electron-emissive material. on the heating element.

9. An indirectly heated cathode structure comprising 5 No references cited. 

